Multiplexing of a bus provides a convenient method of addressing more than one remote device over a single communications channel. The information that travels through the bus is termed the traffic flow of the bus and it contains information that can be utilized by any devices that are in communication with the bus. These devices can either receive information from a central point and/or transmit information from remote locations along the bus to the central position. In addition, some systems allow the remote devices to communicate with each other along the bus. To have an organized traffic flow along the bus, it is necessary to have some type of traffic control that is either located at the central position or distributed among the remote locations. With the advent of large scale integrated circuits, and the central processing units (CPU's ) resulting therefrom, the amount of intelligence available in small packages has greatly increased thus increasing the capability of the remote devices. This has increased the versatility of the applications for multiplex bus systems.
Remote devices that are distributed along a multiplex bus are termed remote terminals (RT's ) and, depending upon the application, these RTs have the capability of both processing information and controlling the traffic flow along the bus. In order for a multiplex system to operate, each device attached to the bus is assigned a unique address. This allows the RT to monitor the bus at all times and respond only when it is addressed. In addition, this allows one RT to communicate with a second and different RT along the bus that is located at a different position. These addresses are normally encoded in a binary format and thus require a corresponding binary address word, or part of a word, to address a specific RT. Parts of words which are reserved for specific functions are called fields. The number of RTs that can be connected to a single bus is limited only by the length of the binary word or address field that is allowed on the bus. For example, a four bit binary word or address field provides a multiplex bus with the capability of addressing up to 16 RTs; therefore, only 16 RTs are permissible for attachment to the bus at any given time. In a similar manner, an infinitely long digital word or field can address an infinite number of RTs. Most time division multiplex bus systems do not allow simultaneous transmissions on the bus so a bus controller (BC) is required that ensures an orderly traffic flow along the bus.
Due to the desire of the electronic industry and the military to standardize systems, limitations have been imposed upon certain operational characteristics of the systems. One of the operational characteristics that has been limited is the number of RTs or addresses available on a given bus. One type of standardized bus is MIL-STD-1553B. This specification limits the address field on the multiplex bus to five bits, thereby limiting the number of addresses on the bus to 32. However, one of these addresses is reserved for common addressing of all terminals which is called Broadcast Mode and thus limits the number of RTs that can be attached to the bus to 31. Although the MIL-STD-1553B multiplex bus provides an industry standard, it also presents a design limitation that must be contended with in each systems application. Providing a capability for only 31 RTs presents a disadvantage in some system applications, such as an airborne control system that would benefit from having more than 31 terminals.
In view of the above limitations with standard multiplex bus systems, it is desirable to develop a system that expands the multiplex bus and thus utilizes the bus controller to its full capability.